High current cold cathode gas triode control system



9, 1966 e. H. REILING 3,265,929

HIGH CURRENT COLD CATHODE GAS TRIODE CONTROL SYSTEM Filed Dec. 19, 1963 2 SheetsSheet 1 Fig \%2 \I I2 20a Fig, 3.

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HIGH CURRENT COLD CATHODE GAS TRIODE CONTROL SYSTEM Aug. 9, 1966 2 Sheets-Sheet 2 Filed Dec. 19, 1963 /m/enf0r Gi/b an H. Ref/mg His Affomey United States Patent 3,265,929 HIGH CURRENT C(Z LD CATHUDE GAS TRIODE CONTRGL SYSTEM Gilbert H. Railing, Chardon, ()hio, assignor to General Electric Company, a corporation of New York Filed Dec. 19, 1%3, Ser. No. 331,874 4 Claims. ((11. 315-199) The present invention relates to an improved static rectifier using a cold cathode space discharge device in which a pilot arc is maintained on an essentially continuous basis and a main arc is established periodically and for a controllable period in the A.-C. cycle to supply predetermined power to the load, such as a D.-C. motor. The apparatus of the present invention is characterized by low energy loss making possible an effective overall unit without elaborate cooling equipment.

Rectifying action in the apparatus herein described is provided at least in part by one or more space discharge devices in which a holding or pilot arc discharge is continuously maintained (after initial ignition) between the cathode and a holding anode. This arc is initially established by an external electrode that receives a momentary high voltage that produces an electric field within the device effective to create conduction carriers. These conduction carriers give rise to the pilot arc discharge between the cathode and holding anode under the influence of a comparatively low voltage thereafter continuously applied to the latter. Means responsive to the resultant pilot arc current flow serves to disable the voltage supply to the starting electrode. The space discharge device is thus conditioned for controlled current carrying rectifying action.

Further in accordance with the present invention, sharply peaked voltage pulses are applied to the control electrode of the discharge'device or devices and at controllable predetermined times in the A.-C. applied voltage wave. These pulses serve to initiate the main arc discharge, causing cur-rent flow pulses through the motor or other load circuit. As hereinafter described, means are provided normally to bias the control electrode to a voltage incapable of producing main arcing current flow. As also described hereinafter, these voltage pulses are of very short duration and do not require substantial \power.

In the apparatus herein described, automatic elements are provided to assure that at all times the pilot or maintaining arc is in fact present. Elements are provided which respond to the interruption of such are to create a high energy striking voltage on the arc-initiating electrode during the first alternating voltage half-cycle following interruption of the pilot arc.. Since this apparatus is effective only when the pilot arc is out, and the pilot arc is normally maintained continuously during rectifying operation, the power requirements of this apparatus are low.

In its preferred form, the apparatus of the present invention is constructed in a full wave polyphase rectifying circuit and the load is in the form of a DC. motor. in this application it is necessary to energize the motor with considerable amounts of power, and to control the power in a way that provides smooth effective speed control despite the counter E.M.F. incident to motor operation. It has been rfound that the apparatus of the present invention is especially effective in this application, it provides adequate and effective speed control, and that it is characterized by such a low power loss that in many installations simple forced air cooling can be employed.

It is an object of this invention to provide an improved, low-loss, controllable rectifier that is not subject to cathode damage during peak anode current or faul-t currents.

Another object of this invention is to provide a rugged 'ice high current cathode which is not subject to emission damage as found in many thermionic (hot cathode) devices.

A further object of the present invention is to provide an improved controllable static rectifier system that is characterized by low power losses.

Another object of the present invention is to provide an improved low-loss controllable static rectifier mechanism that utilizes space discharge devices of the type wherein a holding ipilot arc is. maintained substantially continuously and main arc discharge is achieved at a controllable point in the A.-C. applied wave.

Still another object of the present invention is to provide an improved controllable static rectifier mechanism of the foregoing type wherein the starting anode energization is correlated with the pilot arc current flow to assure that the pilot arc is immediately restarted upon interruption and at the same time unnecessary energization of the starting electrode is avoided.

Yet another object of the present invention is to provide an improved static rectifier mechanism characterized by low energy losses, suitability for cnergization Olf DC. motors, ease of control, simplicity, reliability, and relatively low cost, to the end that a unit having a wide range of commercial utility is provided.

Briefly stated, my invention is directed to a control mechanism for one or more electric or space discharge devices having anode, cathode, control, holding and starting electrodes of the type in which a voltage applied between cathode and holding electrode maintains a pilot arc to facilitate striking of a main are when required for control purposes. In accord with one embodiment of my invention there is provided for each such device a control electrode circuit for supplying voltage to a control electrode to determine when such device is rendered fully conductive; a holding anode voltage circuit to provide for the operation of a pilot discharge; a starting electrode circuit for starting the pilot discharge; and means interconnecting the holding anode circuit and the starting circuit afor restarting the pilot discharge upon accidental extinction thereof, as well as for initial starting.

The novel features believed to be characteristic of the present invention are set forth with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, will best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

FIGURE 1 is an axial cross-sectional view of a space discharge device of the type used in the apparatus of the present invention;

FIGURE 2 is a schematic block diagram showing the circuitry of the apparatus of the present invention as applied to energize a D.-C. motor from a polyphase A.-C. source, and in which part of the rectifying action is provided by the devices of FIGURE 1;

FIGURE 3 is a block diagram of an alternative form of the present invention wherein all rectifier action is achieved through the use of space discharge devices of the type shown in FIGURE 1.

The space discharge device of FIGURE 1 is of the type described and claimed generically in a patent to Reiling and Jensen, Number 2,976,451 entitled Electric Discharge Device. As shown, the device includes an evacuable discharge device body 1 of cylindrical configuration, a mercury-alkali metal cathode 10, an anchor post 11 for immobilizing the cathode spot, a main anode 12, a holding anode assembly 15, a control grid or electrode 20, and an external starting electrode 25.

The body 1 is defined by a tubular ceramic sidewall 2 having one end closed by a base 3, and the other end sealed by a main anode 12 having an inwardly extending pedestal 12a to provide increased surface area for col- 'lecting space-current carriers. The base 3 is provided with a port 3a to facilitate evacuation of gases and admission of cathode material into the interior of the body, and a centrally-disposed threaded recess 3b for removably securing the anchor post 11. The base 3 and main anode 12 may be made of commercial grade titanium, and the cylindrical sidewall 2 may be made of a forsteritic ceramic. The composition of several forsteritic ceramics suitable for use in sidewall 2 are disclosed in United States Letters Patent 2,910,340 issued to A. G. Pincus, November 10, 1959. The forsterites suitable for use in this device are characterized by desirable dielectric properties, freedom from gas-producing constituents, and thermal coefiicients of expansion effectively which substantially match to that of titanium.

The holding anode assembly 15 and the control grid are mounted in transverse relation to the longitudinal axis of side wall 2, and in spacedapart relation with respect to cathode 10, main anode 12, and each other. The holding anode assembly 15 is made up of a holding anode 16 having a terminal 16a and a centrally-disposed circular opening 16b through which the main current carrying arc is established when grid 20 is energized with a positive pulse in relation to the cathode and of sufficient amplitude. A steel baffle 17 is supported on posts 18 and 19 in spaced apart and coaxial relation with respect to the main anode side of opening 16b, thereby obstructing the path of low energy conduction carriers escaping from the pilot arc and passing through the opening enroute to grid 20. Except for opening 16b, anode 16 completely partitions the space within the tube body 1. The baffle 17 eliminates, or at least minimizes, the accumulation of a positive space charge in the vicinity of grid 20, thereby reducing the energy required for effective grid control of the space discharge.

The grid 20 is made of titanium or other suitable material. It is conventiently in the form of a disk having a terminal 20a provided with closely-spaced apertures 20b through which conduction carriers may pass during the conduction period enroute to the main anode 1.2.

The anchor post 11 is made of molybdenum or other refractory metal resistant to cathode sputtering. One end of anchor post 11 is provided with threads mating with those of recess 3b of base 3. As shown, the anchor post 11 defines a dome-shaped face extending above the cathode pool 10.

The cathode 10 is of the cold cathode, metallic liquid type, being composed of mercury and a small percentage of an alkali metal. Although rubidium is preferred, good results also may be obtained with cesium or potas sium or a mixture of the alkali metals. The percentage of alkali metal in the cathode pool may be varied considerably, but is usually less than that which will cause solidification within the range of anticipated starting temperatures. The purpose and function of the alkali metal is described in detail in the above-mentioned Reiling and Jensen application.

The starting electrode comprises a flat metal strip disposed around the external surface of sidewall 2 in a position opposite the face of cathode 10. Application of voltage to the starting electrode 25 in relation to the cathode produces an electric field within the tube as hereinafter described.

Although omitted from the drawings, it should be understood that base 3 and main anode 12 are suitably connected to a source of alternating current, as, for example, by suitable conducting clips. Also, suitable fins (not shown) may be attached to tube body 1 to enhance the dissipation of heat.

The port 3a of base facilitates the introduction of the mercury-alkali metal alloy and the evacuation of air and other gases from the interior of tube body 1. After these operations have been completed the steel sleeve 24 may be crimped and welded to provide a vacuum-tight seal for port 3a.

In fabricating an improved space discharge device of the type represented in FIGURE 1, permanent vacuumtight seals may be effected between the adjoining forsterite and titanium surfaces through the use of known techniques. For example, the sealing methods disclosed in the United States Letters Patent 2,857,663 issued to James E. Beggs, October 28, 1958, may be applied.

A complete static rectifier constructed in accordance with the present invention is shown in FIGURE 2. As shown three rectifiers, 28, 29, and 32, each like the space discharge device of FIGURE 1, are used. Since the starting and control circuitry for each rectifier is essentially the same as the like circuitry provided for the others, the full circuitry is shown only in connection with rectifier 28. For purposes of convenience, the following description is made with reference to the control and energizing circuitry for rectifier 28, it being understood that similar structure and operation is effected by the corresponding cIrcuitry used for rectifiers 29 and 32.

The holding anode voltage supply circuit is indicated generally at 49. It develops a unidirectional voltage of positive polarity for the holding anode 2811 of triode 28. The starting circuit is shown generally at 50. It is energized through normally closed contacts 41k of relay 41, in the holding anode circuit to generate high-voltage, lowcurr-ent, short-duration pulses of positive polarity for starting electrode 28s to start the pilot are between holding anode 28h and the cathode 28c. The grid-control circuit shown generally at 89 provides positive firing pulses to grid 28g at an adjustable time during the successive positive excursions of the alternating voltage applied to main anode 28a and cathode 28c. In addition, it maintains the grid 28g biased to nonconducting condition prior to each firing pulse. The starting circuit 50 is operative only for the very short interval required to energize triode 28. Thereafter this circuit is eifective only if the pilot arc between the holding anode 28a and cathode 28c becomes extinguished, at which time it momentarily energizes as required to restart the holding arc. This selective energization of the starting electrode provides an important economy of operation. For example, if triode 28 is constructed for operation in the range of 30 to 100 amperes maximum main arc current, the power consumed by the holding anode circuit 40 in order to maintain the pilot arc will approximate 30 watts, and the power consumed by the grid control circuit will be about 0.0001 of the power required to fire conventional ignitrons of comparable current rating and in which no holding arc is provided.

Theholding anode circuit The holding anode circuit 40 rectifies the alternating voltage of secondary 44 of transformer 42. The primary 43 of this transformer is energized by the A.C. supply voltage, as shown. The secondary 44 and tap 44m are connected in a conventional half-wave rectifying circuit including rectifiers 45 and 46 to provide negative voltage at filter inductance 48 and positive voltage at the tap 44121, as shown. Inductance 48 and capacitor 49 coact in a filter circuit indicated generally at 47 to provide essentially constant voltage across capacitor 49. As shown, the positive terminal of capacitor 49 is connected to the holding anode 2811 through the winding dlw of relay 41. During the normal operation, the current through winding 41w maintains that relay in the picked up condition, under which the relay contacts 41k are open. The negative terminal of capacitor 49 is connected to the cathode 28c of discharge device 28. The proper constant holding anode voltage is thereby supplied.

Until the pilot arc is established between holding anode 2812 and cathode 28c, the flow of current through the winding 41w of relay 41 is negligible. When the pilot arc is established, the resulting flow of current causes pickup of relay 41, and opens contact Mic. The starting circuit 50 is thus disabled and the pilot arc is maintained 5. by the positive potential supplied to holding anode 2811 by the holding anode circuitry 40.

The holding anode supply circuit may, if desired, be used to supply voltage to the discharge devices, 29 and 32 which, as above indicated, are like device 28. This can be done by connecting the positive terminal of capacitor 49 to the respective holding anodes, as is indicated by the terminals X (connected to capacitor 49), X (holding anode of device 29) and X" (holding anode of device 32). In each instance a relay (not shown) similar to relay 41 is interposed in the circuit and has contacts that are closed when the relay is not energized connected to the starting circuit for the respective discharge device. The respective starting circuits are indicated at 31 and 34 for the devices 29 and 32, respectively, it being understood that each coacts with the relay in the same fashion as is hereinafter described for the starting circuitry 56 of the device 28. The cathodes of the devices are connected together as shown to complete the holding anode voltage supply circuit for each device.

The starting circuit The starting circuit 50 provides high-voltage, low-current, short-duration pulses or spikes to starting electrode 28s for establishing a pilot are between cathode 28c and holding anode 28h of the discharge device 28. The starting pulses develop transient electric fields near the surface of cathode 28c of sufiicient intensity to produce conduction carriers which coact with the field due to the holding anode voltage to strike the pilot arc.

The starting circuit, indicated generally at 59, comprises a transformer 51, a storage capacitor 57, a charging circuit shown generally at 58 for charging storage capacitor 57, an electronic switch 62 effective to discharge capacitor 57 and produce a current pulse in the primary 73 of transformer 72, and an output circuit 70 in which a pulse of high voltage is produced in response to each discharge of capacitor 57.

The transformer 51 has a primary winding 52 connected to voltage input terminals A and N and a multiplicity of secondary windings 53, 54, and 56. The secondary winding 53 provides the operating voltage for the charging circuit 58, secondary winding 55 provides the control voltage for the electronic switch 62, and windings 54 and 56 provide the filament and voltages required for diode and gas discharge tube 63), respectively.

The capacitor charging circuit 58 is made up of resistor 59 and diode 60 in series circuit. It may be traced from the lower end of secondary winding 53 to resistance 59 and thence to the anode 60p of diode 60. The filament (Silk of diode 60 is connected to winding 54, the tap 54111 of which is connected to capacitor 57. The circuit may be further traced from capacitor 57 through primary winding 73 and normally closed relay contacts 41k back to the winding 53. The effect of this circuit is to charge capacitor 57 on each alternate half-cycle of the applied voltage so long as relay contacts 41k are in contact-making condition.

The electronic switch 62 is composed of a thyratron electric discharge device 63 having its anode 63b connected to storage capacitor 57 and its cathode 63k connected across secondary winding 56 of transformer 51 and, through primary winding 73, to capacitor 57. The control electrode 63g of thyratron electric discharge device 63 receives alternating voltage shifted in phase from the voltage of secondary 55. This shifted voltage is derived from R-C phase shifter 66 through resistor 6 which is connected to control electrode 63g. The mid-terminal 55m of secondary winding 55 is connected to cathode 63k through Winding 56 to trigger thyratron 63 at a predetermined delayed time in relation to the voltage across Winding 55. Through the action of the delayed firing voltage applied to control electrode 63g, due to the R-C circuit defined by resistors 64 and 68 and capacitors and 67, the thyratron electric discharge device 63 is fired in predetermined time-delayed relation to the charging of capacitor 57.

The output circuit 70 includes pulse transformer 72 and spark gap 75. As above noted, firing of thyratron electric discharge device 6 3 discharges capacitor 57 in a rapid discharge through the primary winding 73 of transiorrner 72. This induces a high voltage pulse or spike in the secondary winding 74 of pulse transformer 72. The winding 74 is connected to the cathode 28c of device 28 and via spark gap 75, to starting electrode 28s of device 28. The spark gap 75 does not break down until the voltage reaches a predetermined value (associated only with discharge of capacitor 57 through gas tube 63) after which its voltage difierence is low and most of the voltage is applied to the starting electrode 28s.

Each half wave charging pulse on capacitor 57 is at power frequency, that is, 60 cycles. The rate of current change through the primary winding 73 of transformer is accordingly low, and no large voltage is induced in secondary 74. The spark gap 75 accordingly remains nonconducting. For these reasons, charging oi capacitor 57 does not tend to strike the arc in tube 2 8.

It will be noted that the cathode or filament of diode 60 is connected to the same side of capacitor 57 as the anode 63b of the gas discharge tube 63. Thus, thyratron 63 is connected to discharge capacitor 5 7 when changed (as above described) through discharge device 60'. This discharge action is brought about as follows:

The voltage on the control electrode of gas discharge device 66 determines the instant that tube is conducting. This instant is determined by the R-C circuit 66 and is chosen to be at a time when the capacitor 57 has been fully charged. Capacitor 57 thereupon discharges through the anode-cathode space path of device 63, the tapped filament Winding 56 and primary winding 73. The time constant of this circuit is very short, giving a high-value, sharp, current pulse through primary winding 73. The resultant rapidly changing flux in the core of transformer 72 provides a very high induced voltage in secondary 74. This high voltage spike breaks down the spark gap 75 and applies a sufi'iciently high voltage to electrode 28s to initiate the arc in tube 28. The are is thereafter maintained by the holding electrode 2811 which is energized, as above described, from capacitor 49.

At a predetenrnin'ed and adjustable time during each positive halfcycle of the voltage applied to the main anode 28a of cold cathode triode 28, the grid-control circuit generates a firing pulse of positive polarity and sufiicient amplitude to establish the main are between cathode 28c and main anode 28a. :The main arc then persists until a time near the end of the positive half-cycle when the instantaneous voltage ditference between the anode 28a and cathode 28c diminishes below the deionization potential of the complete space path between these electrodes. \In addition, the control circuit 80 provides a unidirectional biasing potential of negative polarity to grid 28g of sufiicient magnitude to prevent establishment of the main arc until a positive firing pulse is applied.

The grid-control circuit 80 is comprised of a peaking transformer 81 having a primary winding 82 and a secondary winding 83, and a biasing circuit comprised of biasing transformer 84 having a primary winding 85, and a secondary winding 86 coupled in parallel with the secondary winding 83 of peaking transformer 81. A semiconductor diode 87 is connected in series circuit with secondary winding 8-3 and capacitor 8%, thus causing capacitor 88 to charge in accord with the voltage of secondary wind-ing 8 6. The rectifier 87 is poled to impart a charge to capacitor 818 that biases control electrode 28g negatively in relation to cathode 280. The value of this voltage is sufiiciently great to prevent the main are from striking in the absence of the positive triggering voltage hereafter described.

The primary winding 82 of peaking transformer 81 is connected to the source of alternating voltage of terminal-s A and N through the variable resistance 90 and capacitor 92. Capacitor 91 provides further control of the phase of the current flow in primary 82 in relation to the A-N voltage and provides a low impedance path for high frequency components of voltages reflected from secondary winding 83.

The transformer 81 is a peaking transformer. That is, it undergoes a very rap-id increase in flux as the applied current increases from a zero value and thereafter magnetically saturates to discontinue the rapid flux increase. Thus, as the voltage across the primary 82 increases (at a time determined by the phase shift in the primary circuit, and hence the adjustment of resistance 90), a very large voltage momentarily appears at secondary 83. This voltage is applied to the control electrode 28g, since the charge on capacitor 88 does not instantaneously change. The control electrode 28g is accordingly energized and the holding arc between electrodes 28c and 28h is extended to the full space path of tube 28, rendering the tube fully conducting.

It is apparent that a change in the adjustment of variable resistor 90 makes it possible to shift the phase of the firing pulses applied to control electrode 28g relative to the positive half-cycles of voltage at anode 2.811. Thus, the instant the discharge device 2 8 is made fully conducting is determined by adjustment of resistor 90. When this instant is advanced (by lowering the value Olf 90) the device 2-8 becomes fully conducting at an earlier point in its cycle. The period of current flow through this dev-iccand the power applied to the load circuit (e.g. motor 35) is increased. Conversely, when the value of variable resistor 90 is changed in the direction of higher resistance the voltage applied to primary 82 is delayed and the instant of firing of tube 2 8 is likewise delayed. This results in a shorter period of conduction of tube 28 and less applied to the load (e.g. motor 35). Where static rectifiers in accordance with this invention are used in a motor control system, the phase of firing pulses generated by the grid-control circuit 80 can be shifted sufficiently to enable control throughout a range extending from about 10 to 100 percent of the rated motor speed.

One of the important applications of static rectifiers is in the field of direct-current motor control. In FIG- URE 2, for example, three static rectifiers in accordance with this invention are utilized in conjunction with three semiconductor diodes 36, 37, and 38 to form a three-phase bridge rectifier for controlling direct current motor 35. As already described above, the first static rectifier is comprised of cold cathode triode 28 and a control unit made up of holding anode circuit 40, starting circuit 50, and grid-control circuit 8th. The second static rectifier includes triode 29, grid-control circuit 30, and starting circuit 31 energized by the second phase source available at input terminals B and N; and the third static rectifier includes triode 32, grid-control circuit 33, and starting circuit 34 energized by the third phase of the voltage source made available at input terminals C and N. Inasmuch as gridcontrol circuits 30 and 33 are essentially like grid-control circuit 80, and starting circuits 3i and 34 are like starting circuit 50, simplification is achieved by representing these circuits in block form.

The operation of the three-phase bridge rectifier of FIGURE 2 is in accordance with well known principles. Although six unidirectional conductive elements are utilized in this circuit, only the conduction periods of the three cold cathode triodes, 28, 29, and 32, are controllable. Because of each branch of the three-phase circuit contains a cold cathode triode, the power supplied to motor 35 can be controlled. Moreover, the use of semiconductor diodes 36, 37, and 38 in lieu of cold cathode triodes reduces the number of control units required and results in an important simplification and cost saving.

The alternative form of three-phase bridge rectifier represented in FIGURE 3 utilizes six of the controllable static rectifiers to provide a controller for motor 100. Three of these rectifiers, 128, 129, and 132 are constructed like the rectifiers 28, 29, and 32, FIGURE 2. They are likewise supplied with similar control circuitry (not shown). The additional three rectifiers 128a, 129a, and 132a correspond to the semiconductor rectifiers 36, 37, and 38, respectively, of FIGURE 2. These are provided with control circuits, similar to 80, 40, and 50, FIGURE 2, and arranged to provide conduction at the proper controlled point in the alternating voltage wave across the respective devices 128a, 129a, and 132a.

Since the control is effected through additional controlled rectifiers in the circuitry of FIGURE 3, the back voltage to be withstood is distributed between them and the apparatus may be operated at a higher voltage than with the more simple circuitry of FIGURE 2.

As described above, an important feature of the apparatus of the present invention is that low energy losses occur during normal operation. This is because the pilot are power requirements are small, as are the power requirements of the control electrodes. While the starting power supply circuitry is called upon to produce large energy pulses when the same is operating, the operation of this circuitry is only momentary and takes place only during initial start of the apparatus or occasionally when one of the pilot arcs goes out. It is accordingly possible in the apparatus to provide the starting circuitry, including the capacitor 57, FIGURE 2, the transformer 72, the contacts 41k, the transformer 51, and the gas discharge device 63, of size and power capacity as to be capable only of the occasional operation that is associated with normal operation of the apparatus. That is, portions of this circuitry may be subject to overheating on continuous operation, but have heat capacity sufiicient to prevent overheating during the occasional momentary operation incident to the apparatus herein described. If desired, a fuse, circuit breaker, or other overload responsive device can be included in the starting circuitry, as for example in circuit with the secondary winding 53 of the transformer 51, so that if a pilot are permanently goes out and does not restart in a predetermined short time Within the capacity of the apparatus, the operation of the starting circuit is discontinued, and overheating thereof, is avoided.

Although each of the triodes of FIGURE 2 is represented as having an independent control unit, it should be understood that the three grid-control circuits, 80, 30 and 33, may be intercoupled mechanically or electromechanically in accordance with well known techniques to provide for their simultaneous operation from a single control device. An arrangement of this type would eliminate the necessity of adjusting each of the gridcontrol units independently in order to Vary the power supplied to motor 35.

It will be noted that the various control circuits above described are connected to differing voltages appearing in the three phase A.-C. supply circuit. This forms a convenient way-in the case of the control or grid electrode circuits and the starting circuits to obtain voltage pulses at the desired instants of time. It will be understood, of course, that voltages may be taken from other points in the circuits, if desired.

From the foregoing, it should be apparent that this invention fulfills an important need for controllable static rectifiers characterized by durability, reliability, efliciency, and comparatively low manufacturing and operating cost for use especially in controllable sources of direct-current power for loads requiring an input current within the range of about 30 to amperes.

The representations in the drawings and foregoing text are intended merely to facilitate the practice of this inven tion by persons skilled in the art, not to restrict its scope. Moreover, it is obvious that many circuit variations and substitutions may be made with respect to the specific embodiment described above while remaining within the 9 scope of this invention as delineated in the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A control mechanism for a first electric discharge device having anode, cathode, control, holding and starting electrodes and in which voltage continuously applied to the holding electrode maintains an are for full space current conduction upon application of positive voltage to the control electrode, the improvement comprising:

(a) means effective continuously to supply arc-maintaining voltage to the holding electrode, said means including a relay having an operating winding responsive to current flow to the holding electrode and a pair of contacts that are open when said current flow is substantially at arc-maintaining value and closed when said current flow is substantially below arc-maintaining value;

(b) a source of alternating voltage;

(c) a capacitor;

(d) a rectifier;

(e) means defining a charging circuit to said capacitor from said source through said rectifier and said pair of contacts, to cause the capacitor to be charged at a particular time of each alternate half cycles when the arc-maintaining current flow is not substantially maintained;

(f). a second electric discharge device;

(g) a transformer having a primary winding and a secondary winding;

(h) means defining a short time constant discharge circuit for said capacitor through said second electric discharge device and the primary winding of said transformer;

(j) means responsive to said source of alternating voltage, including an R-C network in the control electrode circuit of said second electric discharge device and effective to render said second electric discharge device conducting subsequent to said predetermined time each of said alternate half cycles;

(k) and means defining a circuit from the secondary winding of said transformer to the starting electrode of said electric discharge device, said means including a spark gap effective to break down only when said capacitor discharges.

2. A control mechanism for a first electric discharge device having anode, cathode, control, holding and starting electrodes and in which voltage continuously applied to the holding electrode maintains an arc for full space current conduction upon application of positive voltage to the control electrode, the improvement comprising:

(a) means effective continuously to supply arc-maintaining voltage to the holding electrode, said means including a relay responsive to the current flow in the holding electrode;

(b) a capacitor;

(c) circuit means including said capacitor and a resistance defining a short time constant and also including contacts on said relay effective to define a charging circuit for charging said capacitor by a predetermined time of each alternate half cycle when current flow through the relay is less than the arcmaintaining value;

(d) means defining a short time constant discharge circuit for said capacitor, said means including a second electric discharge device and means operative in sequence with the voltage applied to the capacitor and including an R-C network in the control electrode circuit of said second electric discharge device to render the discharge device conducting subsequent to said predetermined time of each alternate half cycle;

(e) a peaking transformer having p rnnary and secondary windings, the primary winding being connected in the discharge circuit of said capacitor;

(f) and means defining a circuit from the secondary winding of said peaking transformer to the starting electrode of said first electric discharge device, said means including a spark gap effective to break down only when said capacitor discharges.

3. A control mechanism for a first electric discharge device having anode, cathode, control, holding and starting electrodes and in which voltage continuously applied to the holding electrode maintains an are for full space current conduction upon application of positive voltage to the control electrode, the mechanism comprising:

(a) means effective continuously to supply arc-maintaining voltage to the holding electrode, said means including a relay responsive to the current fiow in the holding electrode; and,

(b) means capable of only momentary operation effective to supply arc-starting voltage to the starting electrode, said means including a capacitor, means including said relay a second electric discharge device and a time constant circuit therefor effective to charge said capacitor by a predetermined time of each alternate half cycle when current flow from said first means is below arc-sustaining value, a transformer having a primary winding and a secondary winding, means effective to discharge said capacitor in a short time constant discharge through said second electric discharge device and said primary winding and operable upon the charging of an R-C network in the control electrode circuit of said second electric discharge device after said predetermined time of each alternate half cycle when said capacitor is charged, and means including a spark gap defining a circuit from the secondary Winding to said starting electrode, the gap being capable of breaking down only upon discharge of said capacitor.

4. A control mechanism for an electric discharge device having anode, cathode, control, holding and starting electrodes and in which voltage continuously applied to the holding electrode maintains an are for full space current conduction upon application of positive voltage to the control electrode, the improvement comprising:

(a) a source of alternating voltage;

(b) means connecting said source to said device for rectifying action;

(0) a capacitor connected between the cathode and control electrodes of said device;

(d) means effective to charge said capacitor in direction to render the control electrode negative in relation to said cathode, thereby preventing full space current conduction in said device;

(f) a saturable transformer having a secondary winding connected in series circuit with said capacitor and the cathode and control electrodes of said device, said transformer further having a primary winding; and,

(g) means to apply alternating voltage of like frequency as said source but adjustable phase in relation thereto to said primary winding, the value of said voltage being suflicient to saturate said transformer and produce short voltage pulses in the secondary of the transformer which are superimposed on the voltage of the capacitor and impart short positive voltage excursions to the control electrode to initiate full space current conduction in said device.

References Cited by the Examiner UNITED STATES PATENTS 2,020,731 11/1935 Lederer 315-239 2,078,671 4/1937 Knowles 315194 2,333,446 11/1943 Rogers 315194 2,999,961 9/1961 Filberich 315203 JOHN W. HUCKERT, Primary Examiner.

D. O. KRAFT, Assistant Examiner. 

1. A CONTROL MECHANISM FOR A FIRST ELECTRIC DISCHARGE DEVICE HAVING ANODE, CATHODE, CONTROL, HOLDING AND START ING ELECTRODES AND IN WHICH VOLTAGE CONTINUOUSLY APPLIED TO THE HOLDING ELECTRODE MAINTAINS AN ARC FOR FULL SPACE CURRENT CONDITION UPON APPLICATION OF POSITIVE VOLTAGE TO THE CONTROL ELECTRODE, THE IMPROVEMENT COMPRISING: (A) MEANS EFFECTIVE CONTINUOUSLY TO SUPPLY ARC-MAINTAINING VOLTAGE TO THE HOLDING ELECTRODE, SAID MEANS INCLUDING A RELAY HAVING AN OPERATING WINDING RESPONSIVE TO CURRENT FLOW TO THE HOLDING ELECTRODE AND A PAIR OF CONTACTS THAT ARE OPEN WHEN SAID CURRENT FLOW IS SUBSTANTIALLY AT ARC-MAINTAINING VALUE AND CLOSED WHEN SAID CURRENT FLOW IS SUBSTANTIALLY BELOW ARC-MAINTAINING VALUE; (B) A SOURCE OF ALTERNATING VOLTAGE; (C) A CAPACITOR; (D) A RECTIFIER; (E) MEANS DEFINING A CHARGING CIRCUIT TO SAID CAPACITOR FROM SAID SOURCE THROUGH SAID RECTIFIER AND SAID PAIR OF CONTACTS, TO CAUSE THE CAPACITOR TO BE CHARGED AT A PARTICULAR TIME OF EACH ALTERNATE HALF CYCLES WHEN THE ARC-MAINTAINING CURRENT FLOW IS NOT SUBSTANTIALLY MAINTAINED; (F) A SECOND ELECTRIC DISCHARGE DEVICE; (G) A TRANSFORMER HAVING A PRIMARY WINDING AND A SECONDARY WINDING; (H) MEANS DEFINING A SHORT TIME CONSTANT DISCHARGE CIRCUIT FOR SAID CAPACITOR THROUGH SAID SECOND ELECTRIC DISCHARGE DEVICE AND THE PRIMARY WINDING OF SAID TRANSFORMER; (J) MEANS RESPONSIVE TO SAID SOURCE OF ALTERNATING VOLTAGE, INCLUDING AN R-C NETWORK IN THE CONTROL ELECTRODE CIRCUIT OF SAID SECOND ELECTRIC DISCHARGE DEVICE AND EFFECTIVE TO RENDER SAID SECOND ELECTRIC DISCHARGE DEVICE CONDUCTING SUBSEQUENT TO SAID PREDETERMINED TIME EACH OF SAID ALTERNATE HALF CYCLES; (K) AND MEANS DEFINING A CIRCUIT FROM THE SECONDARY WINDING OF SAID TRANSFORMER TO THE STARTING ELECTRODE OF SAID ELECTRIC DISCHARGE DEVICE, SAID MEANS INCLUDING A SPARK GAP EFFECTIVE TO BREAK DOWN ONLY WHEN SAID CAPACITOR DISCHARGES. 